1. Field of the Invention
This invention relates to a knock control device for an internal combustion engine in which the ignition timing of the engine is retarded in response to occurrence of knock, and more particularly to such a knock control device in which the exhaust system of the engine is prevented from being heated to an abnormally high temperature when the ignition timing is retarded in response to occurrence of knock.
2. Description of the Prior Art
As is well known in the art, it is generally desired that the ignition timing of an internal combustion engine be set at a point to obtain so-called MBT (Minimum advance for Best Torque) from the points of engine efficiency and fuel consumption. This point will be simply referred to as the "MBT point" hereinafter. However, if the ignition timing is advanced toward the MBT point under certain engine conditions, knock occurs before the MBT point is reached. Knock is apt to occur when the engine is operated at a low speed and under a heavy load, especially in an engine provided with a supercharger. Knock can be prevented by setting the ignition timing at a point immediately behind the knocking limit. However, since the knocking limit depends upon not only the ignition timing but also the air to fuel ratio and/or the atmospheric conditions such as the environmental temperature, the ignition timing has usually been set at a point substantially behind the MBT point in order to ensure that knock does not occur anywhere over the entire range of operational conditions. Accordingly, for some operational conditions the retardation of the ignition timing becomes much more than necessary for preventing knock, which is not desirable from the points of engine efficiency and fuel consumption.
Thus, there has been proposed a system in which the ignition timing is set immediately behind the knocking limit and means for suppressing knock is actuated when a knock detecting means detects occurrence of knock.
When knock occurs in an internal combustion engine, the knock is typically suppressed by reducing the charging efficiency by closing the throttle valve or, in case of an engine provided with a supercharger, by lowering the supercharging pressure; by lowering the air to fuel ratio; in case of an engine provided with an EGR (Exhaust Gas Recirculation) system, by increasing the amount of the recirculation exhaust gas; or by retarding the ignition timing. The last method is advantageous over the other three methods in view of the quickness in response. The other three methods are slow in response since they control flow of a fluid, i.e., the exhaust gas or the air/fuel mixture.
Various systems for carrying out the last method are disclosed in Japanese Unexamined Patent Application No. 56(1981)-27066, U.S. Pat. No. 4,236,491 and U.S. Pat. No. 4,282,841, for example, and generally comprise an ignition timing controlling circuit which detects the rotational speed of the engine or the load imparted thereto to determine the ignition timing, and a knocking detecting circuit having a knock sensor connected to the ignition timing controlling circuit. The knock detecting circuit generates a retarding signal when the knock sensor detects knock and inputs it into the ignition timing controlling circuit which retards the ignition timing by a predetermined angle upon receipt of the retarding signal. This system is quick in response since no behavior of fluid is involved in the controlling system thereof. As is well known in the art, knock of an engine can be detected from the vibration or sound of the engine, and the knock sensor may comprise, for example, a piezoelectric element which detects occurrence of knock through vibration of the engine.
However, said system is disadvantageous in that when the ignition timing is retarded, the combustion gas is apt to burn in the exhaust manifold after being discharged from the combustion chamber, that is, an after-burning phenomenon is apt to occur, whereby the temperature of the exhaust gas is significantly elevated. If the temperature of the exhaust gas becomes abnormally high, there arises a possibility of the exhaust pipe being damaged, the catalyst for cleaning the exhaust gas being fused or, if the engine is provided with a supercharger, the supercharging turbine being damaged.